Unless I misunderstand the question, that sounds like a basic sample loop where playback starts at the beginning and continues until it hits the loop end where it returns to the loop start and continues looping until there's a noteoff/cut/fade.
If that's not what you're talking about, what are some more details in what you're looking for? For example, Impulse Tracker (I love IT

) has sample loops, sample sustain loops (very cool stuff), instrument loops, and instrument sustain loops (also very cool). Most modern trackers will support both types of loops using forward and ping-pong looping.